The purpose of this study is to identify gender specific movement patterns that may predispose females to non-contact ACL injuries during sport activities, and to quantify the effects of a training program in altering 3otentially injurious lower extremity mechanics. In addition, an evaluation of the mechanical and neuromuscular demands of specific exercises included in a training program will be conducted. Three projects have been proposed to address these goals. The objective of Project 1 is to quantify gender differences in biomechanical and neuromuscular aspects of sport specific movements, and to determine how gender specific movement patterns vary across different stages of maturation. Eighty female and 80 male athletes will undergo biomechanical testing while performing cutting and landing tasks. It is hypothesized that after the onset of puberty, significant gender differences will be found at the hip and knee and that these differences will be suggestive of "at risk behavior" with respect to ACL injury. The objective of Project 2 is to assess the influence of a training program in altering biomechanical and neuromuscular aspects of movement in female athletes, and determine whether a given response to a training program varies across different stages of maturation. One-hundred and eighty female athletes (90 intervention and 90 controls) between the ages of 12-24 will participate. Subjects in the intervention group will undergo biomechanical testing before and after participation in a 12 week exercise program that has been found to decrease the incidence of ACL injuries. It is hypothesized that following the training program, females will demonstrate movement patterns that are less suggestive of "at risk behavior." The objective of Project 3 is to assess the mechanical and neuromuscular demands of specific exercises that are included in an injury prevention program that has been shown to reduce the incidence of ACL injuries. Biomechanical assessment of fifteen female athletes performing eight specific exercises will be carried out. It is hypothesized that the specific demands imposed by certain exercises within the intervention program will correspond to changes in hip and knee mechanics following participation in the program. Taken together, data from this study will address important questions related to the influence of gender, age and training on ACL injury potential, and will provide a foundation on which further investigation of injury prevention and rehabilitation can be based.